Modern digital circuitry operates at very high clock speeds to process data and other signals. The rapid rise times which characterize modern digital equipment gives rise to a substantial amount of broadband radio frequency radiation. If this noise is allowed to exit the chassis enclosure or cabinetry it will cause electromagnetic interference (EMI) with other nearby electrical equipment. In general, modern digital equipment is designed to comply with EMI radiation specifications that have been adopted by industry to ensure reliable operation of digital equipment. Although a complete Faraday type enclosure is the most desirable form of electromagnetic shielding for an enclosure, practical devices require both input and output ports to interact with their environment as well as cables associated with power and the like. Accommodating these “EMI holes” represents a continuing challenge for enclosure designers.
In the prior art, the most common enclosure strategy is to fabricate the cabinetry from sheet metal. Typically, metal panels are stamped and bent into appropriate shapes and can be assembled by welding or assembled by other mechanical means. The metallic panels are electrically conductive and can form a complete conductive enclosure that sometimes is welded together, a process that melts and fuses the panels and results in excellent electrical connections between the enclosure panels. Other methods of assembly may utilize rivets, screws and nuts, that employ conductive gasketing between the sheet metal joints. All these attributes along with the ductility of metal allows the formation of an enclosure that can contain higher levels of emanations.
In addition, when the chassis enclosure is open for maintenance, portions of the digital circuitry within the enclosure are not reliably grounded. In this state electronic components can be damaged by electrostatic discharge (ESD). In the prior art metal chassis, several receptacle connections are provided. These connections are used for coupling grounding straps or other ESD suppression devices to the service personnel. Attaching these “receptacles” to the conventional grounded metal enclosure in the prior art is relatively straightforward. However, if not appropriately managed exterior metal components, such as ESD connectors and lift handles, present special EMI problems. These components can form antennas for radiating EMI if not properly and reliably grounded.
In spite of the advantages of conventional metal enclosure solutions there is a continuing need to improve the EMI shielding of electronic equipment and there is a continuing desire to build new and useful enclosures that suppress radiation while permitting enclosure, operation and servicing of modern digital communication and computing systems.